Compressed gas is a used throughout many industries. As a result of the compression process, the temperature of the gas is increased and the dew point level of the gas is increased. In order to be usable, however, the compressed gas must be dried and cooled to an acceptable level.
Two types of gas compressors are oil-free rotary screw or centrifugal. A rotary screw type compressor typically utilizes two stages of compression and a centrifugal compressor typically utilizes two to four stages of compression. Each stage of compression consists of a means of increasing the pressure of a given volume of gas. A moisture separator and a heat exchanger. The separator removes moisture which results from the compression process and the heat exchanger reduces the temperature of the newly compressed gas. At the discharge of the final stage of compression a heat exchanger, often termed an “after-cooler”, is provided in order to lower the temperature of the compressed gas to a suitable level. This cooled compressed gas is then provided to a dryer to achieve a suitable dew point level to allow utilization of the compressed gas.
In today's economy, industry strives to find the most energy efficient equipment available. Heat-of-compression dryers have been utilized for many years and are capable of delivering high quality compressed gas with the very low energy consumption. The high level of efficiency provided by heat-of-compression dryers is possible because these dryers utilize the heat which results from the gas compression process. An example of a heat-of-compression dryer is described in U.S. Pat. No. RE 39,122.
As shown in U.S. Pat. No. RE 39,122 the heat-of-compression dryer 20 includes two towers of desiccant 48, 60 and is utilized with a compressor 22. The compressor 22 includes an inter-stage compressed gas outlet which is connected to the dryer 20. The dryer 20 is connected to the compressor 22 immediately downstream of the first stage of compression, providing inter-stage compressed gas to the dryer in order to regenerate desiccant in the dryer 20. The dryer 20 is also connected downstream of the a heat exchanger 28 in order to provide cooled gas to the dryer where the gas is to be dried for consumption. Valves are provided to set different modes of operation of the dryer. In one mode of operation the first tower of desiccant is “on-line” and receives gas from the heat exchanger to dry the gas to the desired dew point prior to utilization. At the same time, the second tower is “off-line” and receives inter stage gas provided by the first stage of compression of the compressor 22. This inter stage gas regenerates the desiccant in the second tower so that the desiccant is available to dry the gas when the second tower goes “on-line”. When the desiccant in the first tower becomes saturated, the valves are utilized to place the second tower “on-line” and to place the first tower “off-line” for regeneration. In certain instances, the inter stage compressed gas does not provide sufficient heat to regenerate the desiccant in the tower. In these instances, use of the heat of compression dryer is impractical.
In order for the heat-of-compression dryer to operate, sufficient heat must be created during the compression stage to regenerate the desiccant. In some instances, sufficient heat is created only at the final stage of compression to regenerate the desiccant. A prior art system which utilizes final stage compressed gas to regenerate the desiccant is illustrated in attached FIG. 1. The system includes a compressor 70 and a heat-of-compression gas dryer 72. The compressor 70 includes a first stage compressor 74, a first stage heat exchanger 76, a second stage compressor 78, and an after-cooler 80. The compressor further includes a source inlet 82, and a compressed gas outlet 84.
The dryer 72 includes a first desiccant tower 86, a second desiccant tower 88, and a heat exchanger 90. The dryer further includes pipes and valves to connect various components of the dryer 72 and to direct the gas flow through the dryer 72 as desired. The dryer further still includes a wet gas inlet 92 and a dry gas outlet 94.
Because gas exiting the after-cooler 80 does not have sufficient heat to regenerate the desiccant in the heat-of-compression dryer, as illustrated in FIG. 1 the pipe 96 connecting the second stage compressor 78 to the after-cooler 80 has been removed and the pipe 98 connecting the after-cooler 80 to the compressed gas outlet 84 has been removed. The removal of these connections renders the after-cooler 80 inactive. The wet gas inlet 92 of the dryer 72 is connected to the compressed gas outlet 84 of the compressor 70 to provide the wet compressed gas from the compressor 70 to the heat-of-compression dryer 72. The desiccant towers 88, 86 are cycled “on-line” and “off-line”. When the first desiccant tower 86 is “on-line”, for example, the hot compressed gas from the final stage of compressor 78 is provided to the heat-of-compression dryer 72 and the valves are opened and closed as necessary to first direct the gas through the second desiccant tower 88 where it is used to regenerate the desiccant in the second/“off-line” tower 88 of the dryer 72. After exiting the second tower 88, the gas is provided to the first/“on-line” desiccant tower 86 where it is dried to an acceptable dew point. As noted above, the gas at the dry air outlet 94 which is provided for consumption must also be provided at an acceptable temperature. The temperature of the gas exiting the second desiccant tower 88 must therefore be reduced. Thus, upon exiting the second desiccant 88 tower and prior to entering the first desiccant tower 86, the hot gas is provided to a heat exchanger 90 which has been incorporated into the heat-of-compression dryer 72 to cool the gas to the appropriate temperature before being passed to the first desiccant tower 86 (i.e. the “on-line” tower) of the dryer. As the gas is passed through the first desiccant tower 86, the gas is dried to the appropriate dew point necessary for downstream consumption at the dryer outlet 94.
Because the compressor 70 is utilized in a variety of systems wherein the after-cooler 80 is required, manufacture of the compressor 70 without the after-cooler 80 is not practical or cost effective. Likewise, removal of the after-cooler 80 associated with the compressor 70 and incorporation of a heat exchanger 90 into the dryer 72 is not only time consuming but also adds significant cost to the system as two heat exchangers 80, 90 are provided but only one heat exchanger 90 is utilized.
In addition to the increased costs, in some instances, removal of the after-cooler from the compressor is not practical due to the controls associated with the compressor. For example, in some instances the air-cooled after-cooler and the first stage heat exchanger of the compressor utilize a fan(s) to remove heat from the compressed gas. The fan(s) are often controlled by a single control signal. As a result, the after-cooler cannot be removed without impacting the controls associated with the fans of the remaining heat exchanger. Another difficulty is that many energy efficient gas compressors, for example rotary screw compressors, utilize variable speed drives to match the speed of the compressor's motor to the required gas consumption/gas flow. A gas flow sensor at the output of the after-cooler senses the volume of gas flow and a control signal is provided in response to the sensed volume to increase or decrease the drive speed of the compressor's motors thereby matching the increase or decrease in the demand for compressed gas. Because the sensor is located downstream of the after-cooler, the after-cooler cannot be removed without impacting the variable speed drive controls.
A gas drying device which utilizes a cooler to decrease the temperature of the air to be dried is described in U.S. Pat. No. 7,757,407 (“the '407 Patent”) and has been assigned to Atlas Copco Airpower. This system includes two dryers, a cooling dryer 1 and a desiccant dryer 2. The desiccant dryer 2 of the '407 Patent is not a dual desiccant tower dryer. Rather, the desiccant dryer 2 of the '407 Patent includes a pressure tank 4 with a drying zone 5 and a regeneration zone 6 with an adsorption and/or absorption medium 7. The medium 7 is alternately guided through the drying zone 5 and the regeneration zone 6. A portion of the gas compressed by the compressor 3 is provided to the cooling dryer 1 to lower the temperature of the compressed gas and a portion of the gas compressed by the compressor 3 is provided to the desiccant dryer 2 to regenerate the wet medium 7 in the regeneration zone 6. These portions are then mixed together and provided to the medium 7 in the drying zone 5 to lower the dew point of the gas.
In addition, to the fact that the Atlas Copco system incorporates not one but two dryers, because only a portion of “hot” gas provided by compression is utilized to regenerate the desiccant of the dryer 2, a substantial amount of time is required to regenerate the desiccant in the dryer 2 and as a result this system does not provide adequate adsorption to meet the dew points demands required by many compressed gas systems. Furthermore, the portion of the gas provided to the regeneration zone 6 is not cooled prior to flowing through the drying zone 5. The temperature of the desiccant in the drying zone 5 is therefore increased resulting in a decrease in drying efficiency. Finally, the temperature of the gas provided for consumption is increased due to the mixture of the hot gas exiting the regeneration zone with the cooler gas provided to the drying zone.
Accordingly, a need exists for a heat-of-compression compressed gas dryer system which allows for the final stage compressed gas to be provided to the dryer in order to regenerate the desiccant, which provides for cooling of the entire flow of compressed gas prior to drying, and which does not negatively impact the controls associated with the compressor.